const particle_canvas = $("#particle-canvas");
const particle_ctx = particle_canvas.getContext("2d");

let last_time;
let particles = new Set;

update_particle_canvas_size();
window.requestAnimationFrame(particle_loop);

function update_particle_canvas_size() {
    particle_canvas.width = document.body.clientWidth;
    // Whatever's bigger, we use.
    particle_canvas.height = Math.max(document.body.clientHeight, window.innerHeight);
}

function spawn_particle(x, y, vel_x, vel_y, rot, vel_rot, img, w, h) {
    particles.add({
        x: x,
        y: y,
        vel_x: vel_x,
        vel_y: vel_y,
        rot: rot,
        vel_rot: vel_rot,
        img: img,
        w: w,
        h: h
    });
}

function update_particles(delta_time) {
    particle_ctx.clearRect(0, 0, particle_canvas.width, particle_canvas.height);
    for (particle of particles) {
        // Delete off-screen particles, more or less. For now, it's
        // designed so that particles go down with the force of
        // gravity, so we just check if the particle goes below the
        // screen.
        if (particle.y > particle_canvas.height) {
            particles.delete(particle);
            continue;
        }

        // Update the position and rotation of the particle.
        particle.x += particle.vel_x * delta_time;
        particle.y += particle.vel_y * delta_time;
        particle.rot += particle.vel_rot * delta_time;
        // Update the velocity to take into account "gravity".
        particle.vel_y += 0.001 * delta_time;
        // Render particle.
        particle_ctx.save();
        particle_ctx.translate(particle.x + particle.w / 2, particle.y + particle.h / 2);
        particle_ctx.rotate(particle.rot * Math.PI / 180);
        particle_ctx.drawImage(particle.img, -particle.w / 2, -particle.h / 2, particle.w, particle.h)
        particle_ctx.restore();
    }
}

function particle_loop(timestamp) {
    // Calculate the times needed for updating.
    update_particles(timestamp - last_time);
    last_time = timestamp;
    window.requestAnimationFrame(particle_loop);
}